This invention relates to measurement techniques and, in particular, to measurement techniques for determining the moisture content of materials.
In the manufacture of a wide variety of products, it is often desired that the moisture content of the materials being processed be measured. In recent years, there has been an interest in utilizing electrical energy for making these measurements.
In some moisture measurement systems employing electrical energy, the energy is at microwave frequencies. U.S. Pat. No. 3,360,721 discloses one such microwave measurement system designed to detect the moisture content of tobacco. In this system, microwave energy at a level set by a variable attenuator is coupled to a microwave transmitter or horn. The transmitted energy passes through a tobacco sample holder to a microwave receiver also in the form of a horn. A precision variable attenuator follows the receiver and couples the received energy to a detector. The precision variable attenuator is set to provide a maximum detected signal in the absence of a tobacco sample in the holder. Subsequent introduction of a sample causes a reduction in detected signal which is then compensated for by a change in the setting of the precision variable attenuator. This change in setting equals the attenuation added to the system by absorbtion of the microwave energy by the tobacco sample and corresponds to the moisture content of the tobacco.
In another microwave system disclosed in British Specification No. 1,376,747, microwave energy from a source is split and coupled into two similar waveguides. Each waveguide includes two waveguide sections, one of which has an opening for receiving a cigarette rod. Ratios of the outputs of the waveguide sections are taken and these ratios used to determine the relative masses of tobacco and moisture in the tobacco rod. Another system in which a ratio of microwave energies is taken is that of U.S. Pat. No. 3,498,112 used for determining the moisture content of fuel oil. In this system, the energy from a microwave power source is again split, with one portion of the energy being coupled through an attenuator circuit and the other portion through a sample holder containing no sample. The attenuator circuit is set so that its output when compared in a ratio meter with the output energy from the empty sample holder gives a value of 1. Subsequent placement of an oil sample in the holder, provides a ratio reduction which can then be related to the oil moisture content.
U.S. Pat. No. 3,460,031 discloses a split waveguide moisture probe wherein the moisture content of a paper web is determined by passing the web through the gap between the probe sections. As in the system of U.S. Pat. No. 3,360,721, a variable attenuator at the waveguide output is used to determine the increased waveguide attenuation resulting from the passing paper web. This increased attenuation provides a measure of the web moisture content. This patent further discloses an alternate system for detecting web moisture using a split waveguide resonator probe. In this case, insertion of the web into the resonator gap changes the resonant frequency and Q of the resonator. These changes in these quantities can then be measured and utilized to determine web moisture content directly or as a cross-check against an attenuation measurement.
Other microwave systems utilizing an unbalanced condition in corresponding microwave waveguide sections are disclosed in U.S. Pat. No. 2,798,197 and British Specification No. 1,124,461. U.S. Pat. No. 2,798,197 discloses a microwave bridge circuit in which a sample is situated in one bridge arm and the other bridge arm is provided with a calibrated attenuator and phase shifter which are varied to obtain a balanced condition. The attenuator and phase shifter values are then used to determine the dielectric constant of the sample. In British Specification No. 1,124,461 two of the arms of a hybrid waveguide junction are brought into an unbalanced condition upon insertion in one of the arms of an oil sample whose water content is unknown and differs from that of a reference oil sample in the other arm. This unbalanced condition is measured and provides an indication of the unknown water content.
Other types of electrical energy moisture measurement techniques wherein the electrical energy is at frequencies lower than microwave frequencies have also been proposed. U.S. Pat. No. 3,795,984 discloses a system for measuring the moisture content of a fibrous material, such as, tobacco, by applying the output of a variable frequency oscillator to a tuned circuit having a measuring capacitor between whose plates pass the fibrous material. Such passage of the fibrous material causes a change in the electrical characteristics of the capacitor which result in a shift in the resonant frequency of the tuned circuit. This shift in resonant frequency is detected and used to vary the oscillator frequency until it resides at the new resonant frequency. At this frequency, the output of the tuned circuit reaches a voltage peak which is related to the energy absorbed by the measuring capacitor and, hence, to the moisture content of the fibrous material. This patent further teaches that the latter voltage is used to provide a measure of low moisture contents and that for high moisture contents such measure is derived from a signal representing the change in resonant frequency of the tuned circuit. U.S. Pat. No. 3,777,358 also discloses a system wherein the moisture content of a fibrous material is detected by measuring the peaks in output voltage of a tuned circuit having a measuring capacitor.
Another apparatus for measuring moisture content, this time of freshly prized remoistened tobacco in hogs heads, is taught in U.S. Pat. No. 3,766,471. In this apparatus, an oscillator signal is applied to a probe which is inserted in the tobacco and the component of the current of the probe circuit in phase with the oscillator signal determined. This current is a measure of the resistive component of the probe impedance and provides a measure of the tobacco moisture content.
While the many systems described above have provided some degree of success in moisture measurements, there still remains a need for a system having improved accuracy and reliability, particularly for measuring low levels of moisture content.
It is therefore a broad object of the present invention to provide an improved apparatus and method for moisture measurement.
It is a further object of the present invention to provide an improved apparatus and method for moisture measurement of tobacco.